e U-937 monocyte-like cultured cell, a model for circulating monocytes, nds and internalizes insulin in an analogous fashion to target cells such liver cells. The U-937 monocyte internalizes insulin at a high rate and e addition of monensin to insulin markedly augments down regulation of ceptors. By contrast, the IM-9 lymphocyte which internalizes insulin orly, shows essentially no additional effect when monensin is added. The ug colchicine which inhibits endosomal transfer to lysosomes in hepatocytes s been studied. There is no localization to an additional compartment such the Golgi with respect to processing of an internalized ligand. The ternal route appears to go from the endosome of one or more types to the sosome of one or more types; the recycling process would appear to occur om either of the two membrane-bounded structures.